Bone fractures are one of the most common forms of injury. The second most common transplantable tissue after blood is bone. Each year, approximately 170,000 bone fractures in the United States fail to heal. In order to encourage the bone healing process, bone grafts are often needed. These grafts are external bone tissue taken from the patient or from an external source. Bone taken from the patient can cause problems in the donor site, and bone taken from other sources has a considerable chance of being rejected by the patient's body. Accordingly, there is a need for biocompatible artificial bone substitutes.
The mechanical properties of human bone vary tremendously according to the location and function in the body. The mechanical properties and biodegradation rate of any artificial bone should be tailored to match the properties of the surrounding bone at the damaged site.
Natural bone is a complex nanocomposite matrix comprising collagen fibers and calcium phosphate minerals. Artificial nanocomposites mimicking certain properties of bone have been shown with limited success. A plurality of complex variables has made production of artificial bone a significant challenge.
Poly L-Lactic Acid (PLLA) is a biocompatible and biodegradable polymer that has been used widely for biomedical applications. However, PLLA based composites have had some drawbacks, including low mechanical properties and acidic degradation by-products.
Hydroxyapatite (HA) has many similarities with the calcium phosphates of natural bone, and has been used with limited but promising effect in biomimetic applications.
Although natural ceramic/polymer nanocomposites with certain, limited, functionality have been suggested, many crucial challenges remain to be overcome. One of the ongoing challenges is increasing the mechanical strength without sacrificing the elongation at break (toughness) of composites made up of PLLA and HA. Prior art PLLA/HA composites crack under stress due to the brittle nature of HA and their load-bearing property is less than desirable.
One prior art approach is utilizing biopolymer nanofibres or microcrystals as a reinforcing agent. The incorporation of more flexible reinforcing agents such as cellulose fibers or crystals has been suggested to reduce the composite brittleness. Moreover, the cellulose, such as cotton sourced cellulose microcrystals or any plant-sourced cellulose nanofibres, appears to simulate the collagen fibrils existing in natural bone. However, use of such materials has had significant challenges, related to their hydrophilic nature. Specifically, such materials have low compatibility with polymeric matrices having a hydrophobic nature. This causes poor mechanical and water absorption properties, and difficulty in utilizing them to make a suitable artificial bone.
The main challenge in fabrication of the PLLA/HA/cellulose nanocomposites or previously developed PLLA and HA by other scientists is weak interfacial bonding that appears to inversely affect the reinforcing efficiency of the HA particles. Use of the two together has been taught against—non-polar polymers including PLLA have almost no affinity to the polar reinforcing agents such as HA or cellulose. This is generally because cellulose and HA have a hydrophilic nature, whereas PLLA has hydrophobic characteristics, resulting in significant challenges when interfacial bonding. Moreover, the strong affinity of cellulose crystals to each other encourages their aggregation and sedimentation, which is undesirable when using them in a composite. Another challenge is the undesirable high water absorption properties of natural fibers such as cellulose fibers, resulting in sooner than expected biodegradation, an undesired size change of the implant, and unmet mechanical strength in vivo.
An artificial bone-like composite is desirable, for use in grafting, implants, or other bone replacement or healing therapies.